1. Field of the Invention
This invention relates to a laser power control apparatus for controlling the power of a laser beam emitted from a laser oscillator, and more particularly it relates to a laser power control apparatus for a semiconductor laser device used as a light source in an electrophotographic printing device such as a laser printer or a laser facsimile terminal.
2. Description of the Related Art
In a typical electrophotographic printing device, a surface of a photosensitive body is evenly charged with electricity, and then selectively exposed to a scanning laser beam to form an electrostatic latent image, which is subsequently developed into a visible image by a developing agent attached thereto. The developed image is transferred from the photosensitive body to a sheet of paper. This printing device comprises a light source unit 1 as illustrated in FIG. 1 for exposure of the photosensitive body surface. In this unit 1, a semiconductor laser device 2 is excited under the control of a laser driver to emit a laser beam and irradiate a polygon mirror 6. The polygon mirror is driven by a polygon driver 7 to rotate in the counter clockwise direction. The laser beam is reflected by the polygon mirror 6 and then directed to a reflector mirror 5 via a f.theta. lens 4, which mirror 5 reflects the laser beam and directed to the surface of the photosensitive body. The position of the reflector mirror 6 where the laser beam hits is changed in accordance with the angle of the polygon mirror 6, so that the surface of the photosensitive body is transversally scanned by the laser beam coming from the reflector mirror 5. A start sensor 8 detects a laser beam directed to an end of the photosensitive body from which scanning starts, and then generates a detection signal to be used for excitation control of the semiconductor laser device 2.
FIG. 2 graphically shows the relationship between the drive current supplied to a semiconductor laser device 2 and the power of a laser beam to be emitted therefrom. The laser power is substantially proportional to the drive current beyond the threshold of the laser device 2, which may shift depending on the temperature of the device and the service time. The ratio of the change in drive current to the change in laser power, or differential efficiency .eta., can slightly vary depending on the manufacturing process of the semiconductor laser device 2 between a minimum limit .eta.min and a maximum limit .eta.max. The difference of inclination among the straight lines in FIG. 2 shows this fact. The laser driver 3 is generally designed to compensate any change in the laser power that can be caused by the difference in the condition which the semiconductor laser device 2 operates.
FIG. 3 schematically illustrates the configuration of a conventional laser driver 3 which drives a laser diode 27 used as the semiconductor laser device 2. The printing device includes a control circuit for controlling all of the electrophotographic processing sections and producing print image data. The control circuit includes a CPU 11 which controls the laser driver 3 along with the polygon driver 7 and the start sensor 8 so as to expose the photosensitive body according to the print image data. The CPU 11 initially instructs the polygon driver 7 to rotate polygon mirror 6, instructs the laser driver 7 to drive the laser diode 27 after the rotation of the mirror 6 becomes stable at a preset speed, and then supplies a detection signal from the start sensor 8 and print image data to the laser driver 7.
The laser driver 3 includes a laser excitation controller 12, a laser power controller 13, a drive current setting section ST, a exciting section DR, and a laser power measuring section MS. The laser excitation controller 12 controls the exciting section DR so that the laser diode 27 is continuously made active when the scanning position is located outside the exposing span of the photosensitive body, and made active and inactive according to the print image data when the scanning position is located within the exposing span. The laser power controller 13 designates a level for the drive current to be supplied to the laser diode 27, based on the laser power measured by the laser power measuring section MS. The controller 13 supplies current-designating 10-bit data indicating the level thus designated, to the drive current setting section ST. The section ST changes the drive current in accordance with the laser power controller 13.
The drive current setting section includes first and second D/A (digital/analog) converters 25 and 26 which are assigned to the lower two bits and upper eight bits of the current designating data, and which perform D/A conversion on the lower 8-bit data and the upper 2-bit data. The analog output voltage A1 from the D/A converter 25 is supplied to the inversion input terminal (-) of an operational amplifier 33 by way of another operational amplifier 31 and a resistor 33. The analog output voltage A2 from the D/A converter is also supplied to the inversion input terminal (-) of the operational amplifier 33 by way of still another operational amplifier 34 and another resistor 35. A reference voltage Vref1 is supplied to the non-inversion input terminal (+) of the operational amplifier 33 The output terminal of the operational amplifier 33 is connected to the base of a PNP transistor 37 via a resistor 36. The emitter of the transistor 37 is connected to a power source terminal VM via a resistor 38 and to the inversion input terminal (-) of the operational amplifier 33. The collector of the transistor 37 is connected to the collector of an NPN transistor 41 via resistor 40. The emitter of the transistor 41 is connected to a ground terminal, and the base thereof is connected to the excitation controller 12 via a resistor. A resistors 42 and a laser diode 27 are connected in series with each other between the collector and emitter of the transistor 41. A diode 45 and a series circuit of a resistor 43 and capacitor 44 are connected in parallel with the laser diode 27.
The exciting section DR is constituted by the transistor 41, resistors 42, 43, capacitor 44, and diode 45. Transistor 37 performs an analog operation to generate an output current corresponding to the output voltage of the operational amplifier 33, and the transistor 41 performs a switching operation to selectively supply the output current from the transistor 37 to the laser diode 27 according to a control signal supplied from the laser excitation controller 12.
The laser power measuring section MS is constituted by a photodiode 28 for monitoring the power of a laser beam emitted from the laser diode 27, a current/voltage converter 29 for performing current-to-voltage conversion on the output current from the photodiode 28 to produce a monitor voltage, and an A/D converter 30 for performing analog-to-digital conversion on the monitor voltage from the current/voltage converter 30. The photodiode 28 is formed in the sam module as the laser diode 28.
FIG. 4 shows in greater detail the configuration of the laser power controller 13, and FIG. 5 shows a time chart of the signals generated in the controller 13. The laser power controller 13 comprises a timing controller 14, adders 16, 20, and 24, selector 18, and latch circuits 21 and 22, and data generators 15, 17 and 24 for generating reference data REF1, increment data "1", increment/decrement data "+1" or "-1", for example.
In the laser driver 3 described above, if the input data to the D/A converter 15 is close to "00(H)" or "FF(H)", the adder 20 can be overflowed or underflowed when the monitor voltage is changed. (H denotes hexadecimal notation.) The timing controller 14 checks the carry outputs of the adders 16 and 20 to increase the input data to the D/A converter 26 by "1" when the overflow is occurred in the adder 20 and decrease the same by "1" when the underflow is occurred in the adder 20.
FIG. 6 shows a relationship between the input data to the D/A converter 25 and the input data to the D/A converter 26. The input data to D/A converter 25 is changed in a range from "00(H)" to "FF(H)" and the input data to the D/A converter 26 is changed in a range from "00" to "11", so that the current-designating 10-bit data is increased or decreased by a step change of "1". The laser drive current ILD and the emitter voltage VO of the transistor 37 are represented by the following equations: EQU ILD=(VM-VO)/R38 and EQU VO=(1+R39/R32+R39/R35) Vref1 -R39/R32.multidot.A1.multidot.R39/R35.multidot.A2,
where
VM is the power source voltage, PA1 A1 is the output voltage of the D/A converter 25, PA1 A2 is the output voltage of the D/A converter 26, PA1 Vref1 is the reference voltage supplied to the non-inversion input (+) of the operational amplifier 33, PA1 R38 is the resistance of the resistor R38, PA1 R39 is the resistance of the resistor R39, PA1 R32 is the resistance of the resistor R32, and PA1 R35 is the resistance of the resistor R35.
It will be understood from the equations that the drive current ILD depends on the output voltages of the D/A converters 25 and 26. Since the D/A converter 25 is of an 8-bit type, the following equation must be satisfied in order to linearly change the drive current ILD. EQU R39/R32.multidot.A1:R39/R35.multidot.A2.apprxeq.1:1/256
However, it is difficult to prevent the resistances of the resistors R32, R35, and R39 and the step changes in the analog output voltages of the D/A converters 25 and 26 from being deviated from design values. Therefore, the linearity of the drive current ILD is impaired when the output voltages A1 and A2 are simultaneously changed as overflow or underflow occurs in the adder 20. FIG. 7 shows an overlap provided in the case where (R39/R32.multidot.A1)/(R39/R35.multidot.A2)&gt;1/256 and a blank provided in the case where (R39/R32.multidot.A1)/(R39/R35.multidot.A2)&lt;1/256. If the input data to the D/A converter 25 is constantly close to "00(H)" or "FF(H)", the input data to D/A converter 26 can be repeatedly increased and decreased by "1" upon change in the input data to the D/A converter 25. The drive current ILD extremely changes each time the input data to the D/A converter 26 is updated. This impairs the stability of the laser power.